9/18/17 Today’s Objec3ves • Networking Layers Sept 18, 2017 Sprenkle - CSCI325 1 Discussion • What is the end-to-end argument? Sept 18, 2017 Sprenkle - CSCI325 2 1
9/18/17 Review: Applications Layering in Network Design OS Hardware • Fundamental Ques3on: What services do applica3ons require from the bare hardware? Ø Don’t make each app implement the same func3onality Ø OSs should implement these services/abstrac3ons • Network links hand a frame to the opera3ng system • But what abstrac3on does the applica3on desire? • Do all applica3ons need the same abstrac3on? • What abstrac3ons do intermediate hosts ( routers ) in the network need? Sept 18, 2017 Sprenkle - CSCI325 3 Protocols • Protocol : An agreement between two “par3es” as to how informa3on is to be transmi]ed Ø More valuable with more users Ø How do we introduce new protocols if everyone must agree? • Network protocols are typically implemented in so`ware Ø Adds overhead to communica3on Ø Network bandwidth vs. Applica3on throughput Ø Small ma]er of code to deploy new protocol • Examples Ø Internet Protocol (IP): global packet transmission/addressing Ø Transmission Control Protocol (TCP): reliable byte stream Sept 18, 2017 Sprenkle - CSCI325 4 2
9/18/17 The Theory: OSI (Open Systems Interconnec3on)Model Func9on Example Ul3mate data Web browser Applica3on Applica3on des3na3on Format Presenta3on Presenta3on ASCII/XDR conversion Interac3on across Restartable file Session Session presenta3on transfer Reliable, ordered Transport Transport TCP delivery Rou3ng/ Network Network IP Internetworking Data framing Data link Data link Ethernet, WiFi over links Bits on Physical Physical SONET, 100BT the wire Each layer adds some more data/bits Sept 18, 2017 Sprenkle - CSCI325 5 OSI Model Func9on Ul3mate data Applica3on des3na3on Format Presenta3on Where does security go? conversion Interac3on across Session presenta3on Reliable, ordered Transport delivery Rou3ng/ What about reliability? Network Internetworking Data framing Data link over links Bits on Physical the wire Sept 18, 2017 Sprenkle - CSCI325 6 3
9/18/17 OSI Model Discussion • OSI standardized before implemented • On the other hand… Internet Engineering Task Force Ø IETF philosophy: “We reject kings, presidents, and vo3ng. We believe in rough consensus and working code” Ø IETF requires two working/interoperable versions before considering a standard • Modular design but some boundaries are arbitrary Ø Why seven layers? Ø What exactly is the session layer? Ø Basic network func3onality exists at mul3ple layers Sept 18, 2017 Sprenkle - CSCI325 7 Internet Architecture • IP Hourglass: email Telecollabora3on NFS HTTP rlogin RPC RSVP TCP UDP IP packet radio Ethernet modem ATM PPP 100BT SONET air • Layering not strict Ø Can define new abstrac3ons on any exis3ng protocol Sept 18, 2017 Sprenkle - CSCI325 8 4
9/18/17 Layering in Internet Applica3ons • Bo]lenecks Ø Boundary crossings app user Ø Copies kernel Ø Context switches socket task context • Layering nice way to copy logically consider TCP � TCP protocols recv sw interface Ø May not lead to fastest IP implementa3on copy Ø But! Processors are Packet � ether hw interface gemng faster… people arrives are more expensive A li]le OS Sept 18, 2017 Sprenkle - CSCI325 9 Layering in Network Design • Encapsulate each layer in lower-layer format • Ethernet’s view of the packet: Header Payload • IP View: Reality: Header Payload Eth IP TCP HTTP Payload • TCP View: Header Payload Sept 18, 2017 Sprenkle - CSCI325 10 5
9/18/17 OSI Layering: HTTP Example Communication abstraction Payload : data to be sent Application Application HTTP Payload HTTP Payload Presentation Presentation Session Session Transport Transport TCP HTTP Payload TCP HTTP Payload IP TCP HTTP Payload Network Network IP TCP HTTP Payload ETH IP TCP HTTP Payload Data link Data link ETH IP TCP HTTP Payload Physical Physical Sender Receiver Each protocol adds its own headers, metadata to facilitate delivery Sept 18, 2017 Sprenkle - CSCI325 11 Datalink Layer (Layer 2): Ethernet/WiFi • Ethernet frame format Error Length/ Preamble Dst MAC Src MAC DATA Check Type Addr Addr IP TCP HTTP Payload Sept 18, 2017 Sprenkle - CSCI325 12 6
9/18/17 Network Layer (Layer 3): Internet Protocol (IP) • Service mode: best effort Ø No guarantees about reliable, in-order, or error-free delivery Ø Enables IP to “run over anything” (e.g., any type of network) Version HLen TOS Length Identification Flags Offset TTL Protocol Header Checksum Source IP Addr Destination IP Addr Options (variable) Pad (variable) Data TCP HTTP Payload Sept 18, 2017 Sprenkle - CSCI325 13 IP Address Alloca3on • Originally, 4 address classes Ø A: 0 | 7 bit network | 24 bit host (1M each) Ø B: 10 | 14 bit network | 16 bit host (64K) Ø C: 110 | 21 bit network | 8 bit host (255) Ø D: 1110 | 28 bit mul3cast group # • Assign network # centrally, host # locally • W&L probably has class B address Ø Prefix = 137.113 = 10001001. 01110001 Sept 18, 2017 Sprenkle - CSCI325 14 7
9/18/17 End-to-End Principle Summary • Argument evolved into a principle • Idea: Keep the internet (middle) simple, end points can be complex Ø Right abstrac3on à Powerful! • IP protocol allows a variety of applica3ons to be built on top of it Sept 18, 2017 Sprenkle - CSCI325 15 EXAMPLE: WHAT HAPPENS WHEN YOU CLICK ON A WEB LINK? Sept 18, 2017 Sprenkle - CSCI325 16 8
9/18/17 What Happens When You Click on a Web Link? www.google.com www.google.com Your computer ? Internet Sept 18, 2017 Sprenkle - CSCI325 17 Steps in Retrieving a Web Page 1. Extract hostname from URL Ø h]p:// www.google.com /foo è www.google.com 2. Use DNS to translate www.google.com to IP address Ø IP address used for Internet rou3ng Ø Result: 172.217.9.196 3. Establish a TCP (socket) connec3on to 172.217.9.196 Ø Protocol agreement for browser and server to speak HTTP Ø TCP handles network problems (drops, corrup3on, etc.) Ø TCP layered on top of IP/Ethernet 4. Routers determine efficient path to 172.217.9.196 Sept 18, 2017 Sprenkle - CSCI325 18 9
9/18/17 Different Kinds of Iden3fiers • Domain name (e.g., www.google.com ) • IP Address (e.g., 172.217.9.196) • Ethernet (e.g., 08-00-2b-18-bc-65) Sept 18, 2017 Sprenkle - CSCI325 19 Different Kinds of Iden3fiers • Domain name (e.g., www.google.com ) Ø Global, human readable • IP Address (e.g., 172.217.9.196) Ø Global, works across all networks Ø Some IP addrs are not global (e.g., 192.168…) • Ethernet (e.g., 08-00-2b-18-bc-65) Ø Also known as the Media Access Control (MAC) addres Ø Local, works on a par3cular network Ø BUT each Ethernet address is globally unique! Sept 18, 2017 Sprenkle - CSCI325 20 10
9/18/17 Domain name to IP address: Domain Name System (DNS) Local DNS server Your computer (137.113.81.46) (137.113.118.200) What’s the IP address for www.google.com ? Oh, you can find it at 172.217.9.196 But how do we know how to find 137.113.81.46? Sept 18, 2017 Sprenkle - CSCI325 21 How Do We Know Our DNS Server? • IP cannot be configured into hosts when they are manufactured • DHCP (Dynamic Host Configura3on Protocol) allows for dynamic assignment of IP addresses Ø Admins don’t need to manually configure hosts • When host needs config info, broadcast DHCPDISCOVER Ø DHCP server responds with IP address, Router’s IP address, and DNS server address • But we s3ll need to know the MAC address in order to communicate Ø need to get to the lowest layer Sept 18, 2017 Sprenkle - CSCI325 22 11
9/18/17 IP address to MAC Address: Address Resolu3on Protocol (ARP) (137.113.81.46) (137.113.118.200) Same IP prefix: 137.113 Broadcast: Anyone know the Ethernet address for 137.113.81.46 ? Ethernet Unicast: Yes, I’m at 08-00-2b-18-bc-65 Ethernet Ethernet addresses are only helpful on our local network. � Matching IP address prefixes tell us that we are on the same network! Sept 18, 2017 Sprenkle - CSCI325 23 Finding Google 1. Know Google’s IP address Ø Update IP header of our packet with correct des3na3on IP address 2. Google’s IP address prefix doesn’t match our IP address prefix Ø We are NOT on the same network 3. We must go through our gateway router Ø Router will pass our packet out to the Internet 4. Update Ethernet address of our packet so that des3na3on Ethernet addr is the address of our router Ø IP address never changes! Sept 18, 2017 Sprenkle - CSCI325 24 12
9/18/17 How Does a Packet Get Through the Internet? Routers send packet to next closest point H R 172.217.9.196 137.113.118.200 H R H R R R R H H: Hosts R R R: Routers H Each router uses IP prefix matching to get our packet one step closer to Google. Dest Ethernet addr in packet gets updated with each hop! Dest IP addr never changes… Sept 18, 2017 Sprenkle - CSCI325 25 How Do Routers Know Where to Send Data? • Forwarding tables at each router • Original Internet: manual update • Today: automa3c update based on “cost” Ø Link State Protocol • Broadcast local connec3vity informa3on throughout the network • Maintain table of cost to get to other des3na3ons • Choose path with smallest cost Sept 18, 2017 Sprenkle - CSCI325 26 13
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